Electromagnetic wave absorbing thermally conductive composition and thermosoftening electromagnetic wave absorbing heat dissipation sheet and method of heat dissipation work

ABSTRACT

An electromagnetic wave absorbing heat conductive composition is used to form an electromagnetic wave absorbing heat dissipating article that is placed between a heat generating electronic component which, when operated, generates heat, reaches a temperature higher than room temperature and acts as an electromagnetic wave generating source, and a heat dissipating component. The composition is non-fluid at room temperature prior to operation of the electronic component, but acquires a low viscosity, softens or melts under heat generation during operation of the electronic component, to fluidize at least a surface of the composition so that the composition substantially fills any gaps between the electronic component and the heat-dissipating component.

TECHNICAL FIELD

The present invention relates to electromagnetic wave absorbing heatconductive compositions for forming electromagnetic wave absorbing heatdissipating articles that are placed between heat generating electroniccomponents which, when operated, generate heat, attain a temperaturehigher than room temperature and can become electromagneticwave-generating sources, and heat dissipating components such as heatsinks and circuit boards so as to cool the electronic components by heatconduction and absorb the electromagnetic waves. The invention relatesalso to thermosoftening electromagnetic wave absorbing heat dissipatingsheets produced from such compositions, and to a method of installingsuch compositions for the purpose of heat dissipation.

BACKGROUND ART

Circuit design for the latest electronic equipment, including TVs,radios, computers, medical devices, office equipment andtelecommunications devices, has become increasingly complex. Forexample, integrated circuits which contain the equivalent of severalhundreds of thousands of transistors are now manufactured for these andother kinds of equipment. This rise in design complexity has beenaccompanied by a parallel trend toward the fabrication of ever smallerelectronic components. That is, manufacturers are finding ways to fitlarger numbers of such components on steadily shrinking devicefootprints while at the same time continuing to reduce the dimensions ofthe device.

Because heat generated by the various working components causes devicemalfunctions and inoperability, methods for effectively dissipating heatgenerated by electronic components are needed.

Such problems associated with heat generation are exacerbated by theincreasing levels of integration being achieved in electroniccomponents—particularly central processing units (CPUs), drivers,integrated circuits (ICs), memories and other large-scale integration(LSI) devices—which are used in electronic equipment such as personalcomputers (PCs), digital video disks (DVDs) and cell phones. Anothercurrent trend is a shift toward higher operating frequencies for higherperformance. This has led to the generation of harmful electromagneticwaves which can cause failure, malfunction or inoperability due toelectromagnetic interference between electronic components, and may havedeleterious effects on the human body.

Many heat dissipating methods, as well as heat dissipating articles andcompositions used in such methods, have already been devised to reducethe heat generated by electronic components. Heat sinks in the form ofplates made of brass and other high thermal conductivity metals are usedin electronic equipment to hold down the rise in temperature ofelectronic components therein during use. These heat sinks carry awayheat generated by the electronic components and release that heat fromsurfaces by means of a temperature difference with outside air.

For heat generated by an electronic component to be efficientlytransferred to a heat sink, it is necessary that the heat sink be placedin close contact with the electronic component. Because of heightdifferences among various electronic components and component tolerancesin the assembly process, a flexible heat conductive sheet or a heatconductive grease is often placed between the electronic components andthe heat sink so that heat transfer from the electronic components tothe heat sink takes place through the heat conductive sheet or grease.Heat conductive sheets made of materials such as heat conductivesilicone rubber are used for this purpose, but a problem with suchsheets is their interfacial thermal resistance.

Methods that have been proposed for lowering the interfacial thermalresistance include the use of heat conductive greases andthermosoftening sheets such as those described in JP-A 2000-509209.However, these prior-art greases and sheets serve only as heatdissipating articles, and lack the ability to absorb electromagneticwaves.

Many attempts have been made to shield out electromagnetic wavesgenerated from electronic components. Such efforts have generallyinvolved the use of metals, platings or electrically conductivecompositions, but these materials all rely on the ability to reflectelectromagnetic waves. Sheets composed of an organic rubber medium,especially chlorinated polyethylene, which is loaded with a softmagnetic powder or ferrite as the electromagnetic wave absorbingconstituent are already available on the market. Yet, such sheets, whilehaving an electromagnetic wave shielding ability, are ineffective forheat dissipation.

Materials endowed with both the ability to conduct heat and the abilityto absorb electromagnetic waves have recently been described in the art.For example, JP-A 11-335472 discloses that sheet-like structuresfabricated from a matrix material such as silicone gel that contains aferrite (e.g., Mn—Zn ferrite, Ni—Zn ferrite) have electromagnetic noisesuppressing effects. However, because such sheets are loaded with anelectromagnetic wave absorbing filler, they are rigid. Moreover, theyhave a low thermal conductivity and are thus poorly suited for use asheat dissipating articles.

One object of the present invention is to provide electromagnetic waveabsorbing heat conductive compositions endowed with both an excellentability to dissipate heat and outstanding electromagnetic wave absorbingproperties which suppress the generation of electromagnetic noise.Another object of the invention is to provide thermosofteningelectromagnetic wave absorbing heat dissipating sheets formed from suchcompositions. An additional object is to provide a method of installingsuch compositions for the purpose of heat dissipation.

DISCLOSURE OF THE INVENTION

As a result of extensive investigations carried out in order to achievethe above objects, we have found that the interfacial use between a heatgenerating electronic component and a heat dissipating component of anuncured composition which contains an electromagnetic wave absorbingfiller, is a solid at normal temperatures, and thermosoftens, acquires alow viscosity or melts in a given temperature range so as to enableready formation into a sheet or other necessary shape offers a number ofadvantages. Such compositions can easily be installed on and removedfrom electronic components and heat sinks. They soften under the effectof heat generated during operation of the electronic components,reducing interfacial contact thermal resistance and thereby improvingthe heat dissipating performance. Moreover, they have excellentelectromagnetic wave absorbing properties which suppress the generationof electromagnetic noise.

That is, we have discovered that the desired electromagnetic waveabsorbing ability and heat dissipation can be achieved by interposing,between a heat generating electronic component and a heat dissipatingcomponent, a composition prepared by selecting a constituent which issolid at normal temperatures but thermosoftens, acquires a low viscosityor melts within a fixed temperature range, and loading this constituentwith a filler having the ability to absorb electromagnetic waves and, ifnecessary, a heat conductive filler.

Accordingly, in a first aspect, the invention provides anelectromagnetic wave absorbing heat conductive composition for formingan electromagnetic wave absorbing heat dissipating article that isdisposed between a heat generating electronic component which whenoperated generates heat, reaches a temperature higher than roomtemperature and acts as an electromagnetic wave generating source, and aheat dissipating component. The composition is characterized by beingnon-fluid in a room temperature state prior to operation of theelectronic component and by acquiring a low viscosity, softening ormelting under heat generation during operation of the electroniccomponent to fluidize at least a surface of the composition so that thecomposition substantially fills any gaps between the electroniccomponent and the heat-dissipating component. In a second aspect, theinvention provides a thermosoftening electromagnetic wave absorbing heatdissipating sheet fabricated from the foregoing electromagnetic waveabsorbing heat conductive composition. In a third aspect, the inventionprovides a method of applying an electromagnetic wave absorbing heatconductive composition. The method is characterized by placing theforegoing composition between a heat generating electronic componentwhich when operated generates heat, reaches a temperature higher thanroom temperature and acts as an electromagnetic wave generating source,and a heat dissipating component; and substantially filling any gapsbetween the electronic component and the heat dissipating component byoperating the heat generating electronic component and generating heat,causing the composition to acquire a low viscosity, soften or melt sothat at least a surface thereof fluidizes, and by also pressing againstthe composition from at least the heat generating electronic componentor the heat dissipating component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a method for measuring noiseattenuation.

FIG. 2 is a diagram showing a method for evaluating tack.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described more fully below.

The electromagnetic wave absorbing heat conductive compositions of theinvention are used in the form of electromagnetic wave absorbing heatdissipating articles disposed at the interface between a heat generatingelectronic component which when operated—particularly by the applicationof a voltage thereto—reaches temperatures higher than room temperatureand can act as an electromagnetic wave generating source, and a heatdissipating component. In an ordinary room temperature state prior tooperation of the electronic component, the composition is non-fluid andis maintained in the state of a sheet or other shaped article or is heldon a base material or the like and thereby maintained in a conveyablestate. Heat generated during operation of the electronic componentcauses the composition to acquire a low viscosity, soften or melt, sothat it substantially fills any gaps between the electronic componentand a heat dissipating component. It is preferable for gaps between theelectronic component and the heat dissipating component to be filledunder the application of a pressing force to the electronic componentand/or heat dissipating component during heat generation by theelectronic component.

The electromagnetic wave absorbing heat conductive compositions of theinvention are composed of an organic binder constituent and anelectromagnetic wave absorbing filler. When additional thermalconductivity is required, it is preferable for the composition toinclude, along with the above constituents, a thermally conductivefiller. Each of these constituents is described in detail below, as isalso a method for preparing the overall composition.

Organic Binder Constituent

Any organic binder constituent may be used as the matrix of theinventive electromagnetic wave absorbing heat conductive composition solong as the composition is substantially solid at normal temperaturesand softens, acquires a low viscosity or melts in a temperature range ofpreferably 40° C. to the maximum temperature reached as a result of heatgeneration by the heat generating electronic component, more preferablyabout 40 to 100° C., and most preferably about 40 to 90° C., so that atleast the surface of the composition fluidizes. Illustrative examples ofsuitable organic binder constituents include substances having a meltingpoint within the temperature range of operation, and preferably 40 to100° C., such as α-olefins, silicone resins and waxes (referred tohereinafter as “low-melting substances”); substances which do not havinga melting point within the above temperature range during operation, butwhich soften or acquire a low viscosity and become fluid at thetemperature during operation (referred to hereinafter as “heat-flowablesubstances”); substances which are syrupy at the above temperature rangeduring operation; and mixtures of thermoplastic resins and/or thermosetresins which melt at a temperature higher than the above-describedtemperature range during operation or have substantially no meltingpoint in combination with the above-described low-melting substances,heat-flowable substances or syrupy substances (in which cases theoverall composition thermosoftens). Mixtures of thermoplastic resinsand/or thermoset resins with low-melting substances, heat-flowablesubstances or syrupy substances are preferred.

It is preferable for the inventive composition to contain, as theorganic binder constituent, one or more polyolefin polymer, acrylicpolymer, fluoropolymer or siloxane polymer. To induce the composition tothermosoften, acquire a low viscosity or melt without liquefaction andrun-off, it is desirable for it to include a low-melting substance, aheat-flowable substance or a syrupy substance. If the composition mustbe fire retarding, it is desirable for it to include a fluoropolymer ora siloxane polymer. Preferred fluoropolymers include liquid fluorocarbonresins, especially hexafluoropropene/vinylidenefluoride/tetrafluoro-ethylene copolymers. Preferred siloxane polymersinclude those like silicone resins which are solids at room temperature,but soften, acquire a low viscosity or melt on heating; and those likealkyl-modified silicones which melt at or above room temperature. Theinclusion of a silicone resin is preferred. Examples of preferredmaterials for maintaining a non-fluid state at normal temperaturesinclude polymers containing RSiO_(3/2) units and/or SiO₂ units,copolymers of such polymers with R₂SiO units (silicone resins), andmixtures of silicone resins and linear polysiloxanes (uncured siliconerubbers, silicone oils). Here, R stands for monovalent hydrocarbongroups.

As noted above, to effect the critical decrease in viscosity, it isdesirable for the composition to contain, for example, an oligomerhaving a relatively low degree of polymerization or a wax. Specificexamples include low-melting substances (e.g., α-olefins, waxes, acrylicoligomers, silicone resins, fluorinated oligomers), heat-flowablesubstances and syrupy substances. The low-melting substances andheat-flowable substances are preferably substances which melt or softenwithin a temperature range of 40 to 100° C.

In the present invention, it is particularly advantageous to mix asubstance which melts in the above-indicated temperature range duringoperation (e.g., the above-described α-olefins, waxes, silicone resins)into a polyolefin-type polymer (preferably ethylene-propylene copolymeror ethylene-propylene-diene terpolymer), acrylic polymer, fluoropolymeror siloxane polymer which does not melt in the above-indicatedtemperature range during operation.

The mixing proportions are not subject to any particular limitation,provided the composition is solid at room temperature, fluidizes underheat generation during operation of the electronic component, and fillsany gaps between the heat generating electronic component and the heatdissipating component. However, it is preferable for the organic binderconstituent to account for 10 to 100 wt %, and especially 20 to 80 wt %,of the composition.

It is advantageous for the organic binder constituent of the inventionto be one which imparts flexibility and tack (essential for temporarilyholding the heat dissipating sheet in place on an electronic componentor heat sink) to the inventive composition. A polymer or other suitablesubstance of a single viscosity may be used, although the use of two ormore polymers or other suitable substances of differing viscosities inadmixture is desirable for achieving a sheet having an excellent balanceof flexibility and tack. The use of two or more such substances ofdiffering viscosity is thus preferred.

It is preferable for the above-described polymer or composition to befirst thermosoftened or melted, then crosslinked because this canenhance the reworkability. That is, initial thermosoftening of thecomposition so as to bring it into close contact with the heatgenerating electronic component and heat dissipating component, followedby crosslinking enable the composition to conform to heat-inducedexpansion and contraction of the components while maintaining a lowthermal resistance. Moreover, when reworkability is required, the factthat the composition is crosslinked enables it to be easily strippedfrom the electronic components and heat dissipating components. It isthus desirable for the composition be curable by a crosslinkingreaction.

To achieve the above ends, it is preferable for the above-describedpolymers to have terminal or pendant curing reactive functional groups.In polyolefin resins and acrylic resins, typical examples of suchfunctional groups include OH, COOH, unsaturated aliphatic groups,glycidyl groups and norbornene groups. In fluoropolymers, the CH moietyon vinylidene fluoride groups may be used for crosslinking. In siloxanepolymers, unsaturated aliphatic groups, silanol groups and alkoxysilylgroups may be used for crosslinking.

Electromagnetic Wave Absorbing Filler

The electromagnetic wave absorbing filler used in the invention ispreferably one or more selected from the group consisting offerromagnetic metal powders and ferromagnetic oxide powders. Aferromagnetic metal powder or a ferromagnetic oxide powder may be usedalone, or both types of powder may be used in admixture.

The ferromagnetic metal powder is preferably iron or an iron-containingalloy. The ferromagnetic iron alloy is at least one selected from thegroup consisting of Fe—Ni, Fe—Co, Fe—Cr, Fe—Si, Fe—Al, Fe—Cr—Si,Fe—Cr—Al, Fe—Al—Si, Fe—B—Si, Ni—Fe and Co—Fe—Ni—Si—B ferromagneticalloys. Any of these ferromagnetic metal powders may be used alone orcombinations of two or more may be used together.

The ferromagnetic metal powder may be composed of particles havingeither a flaky or granular shape, although flaky particles arepreferable for conferring the inventive composition with a good abilityto absorb electromagnetic waves. Because a soft magnetic metal powdercomposed of flaky particles tends to account for a smaller proportion ofthe composition volume on loading, concomitant use can be made of a softmagnetic metal powder composed of granular particles.

If the ferromagnetic metal powder is composed of flaky particles, it isadvantageous for the particles to have an average maximum length of 0.1to 350 μm, especially 0.5 to 100 μm, and an aspect ratio of 2 to 50. Ifthe ferromagnetic metal powder is made of granular particles, it isadvantageous for the particles to have an average particle size of 0.1to 100 μm, and especially 0.5 to 50 μm.

The ferromagnetic oxide powder is preferably a ferrite. Specificexamples of ferrites that may be used include spinel ferrites having abasic composition of ZnFe₂O₄, MnFe₂O₄, MgFe₂O₄, CoFe₂O₄, NiFe₂O₄,CuFe₂O₄, Fe₃O₄, Cu—Zn-ferrite, Ni—Zn-ferrite or Mn—Zn-ferrite; X-typeand Z-type ferrox-planar hexagonal ferrites having a basic compositionof Ba₂CO₂Fe₁₂O₂₂, Ba₂Ni₂Fe₁₂O₂₂, Ba₂Zn₂Fe₁₂O₂₂, Ba₂Mn₂Fe₁₂O₂₂,Ba₂Mg₂Fe₁₂O₂₂, Ba₂Cu₂Fe₁₂O₂₂ or Ba₃CO₂Fe₂₄O₄₁; and M-typemagnetoplumbite hexagonal ferrites having a basic composition ofBaFe₁₂O₁₉, SrFe₁₂O₁₉ and/or BaFe₁₂O₁₉ or SrFe₁₂O₁₉ in which the iron issubstituted with titanium, cobalt, manganese, copper, zinc, nickel ormagnesium. Any one or combinations of two or more of these ferrites maybe used.

The ferromagnetic oxide powder may be composed of particles havingeither a flaky or granular shape, although flaky particles arepreferable on account of their large surface area. Because a magneticoxide powder composed of flaky particles tends to account for a smallerproportion of the composition volume on loading, concomitant use can bemade of a magnetic oxide powder composed of granular particles.

If the ferromagnetic oxide powder is composed of flaky particles, it isadvantageous for the particles to have an average maximum length of 0.1to 350 μm, especially 0.5 to 100 μm, and an aspect ratio of 2 to 50. Ifthe ferromagnetic oxide powder is composed of granular particles, it isadvantageous for the particles to have an average particle size of 0.1to 100 μm, and especially 0.5 to 50 μm.

These electromagnetic wave absorbing fillers are included in an amountof preferably 100 to 3,000 parts by weight, and most preferably 150 to1,600 parts by weight, per 100 parts by weight of the organic binderconstituent. The addition of too little electromagnetic wave absorbingfiller may fail to impart the composition with a sufficientelectromagnetic wave absorbing ability. On the other hand, too much ofthis filler may result in poor fluidity when the composition softens,acquires a low viscosity or melts at the time of heat generation andalso render the composition hard and brittle at room temperature, makingthe composition difficult to form into a sheet.

Heat Conductive Filler

If the formulation of only the foregoing matrix and electromagnetic waveabsorbing filler provides inadequate heat conduction and a greater heatdissipating effect is desired, a heat conductive filler may be usedtogether with the above constituents.

Examples of heat conductive fillers that may be used in the practice ofthe invention include the following substances commonly employed as heatconductive fillers: nonmagnetic metals such as copper and aluminum,metal oxides such as alumina, silica, magnesia, red iron oxide,beryllia, titania and zirconia; metal nitrides such as aluminum nitride,silicon nitride and boron nitride; artificial diamond and siliconcarbide. These heat conductive fillers may be used singly or ascombinations of two or more thereof.

As with the electromagnetic wave absorbing fillers, these heatconductive fillers preferably have an average particle size of 0.1 to100 μm, and especially 0.5 to 50 μm. The particle shape is preferablyround. Use may be made of heat conductive filler having one particleshape or having a plurality of different particle shapes in admixture.To enhance thermal conductivity, it is recommended that particles of twoor more different average particle sizes be blended so as to approach aclosest packing arrangement.

The heat conductive filler is included in an amount of preferably 10 to2,500 parts by weight, and most preferably 1,000 to 2,000 parts byweight, per 100 parts by weight of the organic binder constituent. Toolittle heat conductive filler may fail to provide a sufficient heatconducting ability, whereas too much may detract from the sheetformability of the composition and its ease of use.

Other Additives

The electromagnetic wave absorbing heat conductive compositions of theinvention may optionally include also other constituents, such asadditives and fillers that are commonly used in synthetic rubbers,insofar as this does not compromise the objects of the invention.Specific examples of such additional constituents that may be usedinclude release agents such as silicone oils and fluorine-modifiedsilicone surfactants; colorants such as carbon black, titanium dioxideand red iron oxide; flame retardants such as halogen compounds,phosphorus compounds and platinum catalysts; and processing aids usedwhen formulating conventional rubbers and plastics, such as processoils, reactive silanes or siloxanes, reactive titanate catalysts andreactive aluminum catalysts.

Method of Preparation

The electromagnetic wave absorbing heat conductive compositions of theinvention can be prepared by using a rubber blending apparatus such as atwo-roll mill, Banbury mixer, kneader, gate mixer or planetary mixer,and heating if necessary, to uniformly blend the above constituents.

Thermosoftening electromagnetic wave absorbing heat dissipating sheetscan be produced using a process in which the composition obtained byblending is then formed into a sheet by a suitable technique such asextrusion, calendering, rolling, pressing, or dissolution in a solventfollowed by coating.

The resulting electromagnetic wave absorbing heat conductive compositionand thermosoftening electromagnetic wave absorbing heat dissipatingsheet have a thermal conductivity of preferably at least 0.5 W/mK, andmost preferably 1 to 20 W/mK. At a thermal conductivity of less than 0.5W/mK, the ability to conduct heat between an electronic component and aheat sink or other heat dissipating component decreases so that thecomposition or sheet may fail to exhibit a sufficient heat dissipatingability.

It is preferable for the composition and sheet of the invention to havea viscosity at 80° C. of 1×10² to 1×10₅ Pa·s, and especially 5×10² to5×10⁴ Pa·s. At a viscosity of less than 1×10² Pa·s, run-off of thecomposition or sheet from between an electronic component and a heatdissipating component such as a heat sink may occur. On the other hand,at a viscosity of more than 1×10⁵ Pa·s, the contact thermal resistancemay increase, lowering the ability to conduct heat between an electroniccomponent and a heat dissipating component such as a heat sink. In suchcases, the composition or sheet exhibits an inadequate heat dissipatingability.

Moreover, it is desirable for the above composition and sheet to have aplasticity at 25° C. (JIS K 6200) in a range of 100 to 700, andpreferably 200 to 600. At a plasticity at 25° C. of less than 100,handleability during installation on electronic components may be poor.At a value greater than 700, sheet formability and handleability duringinstallation on electronic components may be poor.

The resulting electromagnetic wave absorbing heat conductive compositionand thermosoftening sheet can easily be installed on and removed fromelectronic components and heat dissipating components such as heatsinks. They acquire a low viscosity, soften or melt under the effect ofheat generated during operation of the electronic components so that atleast the surface of the composition fluidizes, thereby reducinginterfacial contact thermal resistance between the electronic componentsand the heat dissipating components. Moreover, they have an excellentelectromagnetic wave absorbing ability which suppresses the generationof electromagnetic noise.

The above-described composition and sheet are placed between a heatgenerating electronic component which generates heat as a result ofoperation, attains a temperature higher than room temperature andbecomes an electromagnetic wave generating source, and a heatdissipating component. At the time of installation, the composition orsheet is not set in fully intimate contact with the electroniccomponent; instead, small gaps remain. However, heat generated byoperation of the electronic component causes the composition or sheet tosoften, acquire a low viscosity or melt to fluidize at least the surfaceof the composition or sheet so that the composition or sheet fills thesmall gaps and comes fully into intimate contact with the electroniccomponent. As noted above, this has the effect of lowering theinterfacial contact thermal resistance. It is desirable to achieve evencloser contact by applying a pressing force at this time to thecomposition or sheet from at least the electronic component or the heatdissipating component.

The type of heat generating electronic component is not subject to anyparticular limitation, although the inventive composition or sheet iseffective when used with heat generating electronic components thatgenerate electromagnetic waves and heat upon the application thereto ofa voltage, such as those used in personal computers and other electronicequipment.

EXAMPLE

Examples of the invention and comparative examples are given below byway of illustration and not by way of limitation.

Examples 1 to 4

Acrylic-based electromagnetic wave absorbing heat conductivecompositions composed of a mixture of primarily acrylic resin andelectromagnetic wave absorbing fillers and having a softening point ofat least 40° C. were formed as described below into thermosofteningelectromagnetic wave absorbing heat dissipating sheets.

In each case, an acrylic resin was used as the resin constituent of theacrylic-based electromagnetic wave absorbing heat conductive compositionand paraffin wax was used as the thermosoftening constituent. Anotheringredient included in the compositions was carbon functional silane,which was used as a surface treatment agent for the electromagnetic waveabsorbing filler and the heat conductive filler. The starting materialsfrom which the compositions were formulated are listed below.

Starting Materials

-   1) Paraffin wax: Paraffin Wax 115 (melting point, 47° C.) and    Paraffin Wax 130 (melting point, 55° C.) manufactured by Nippon    Seiro Co., Ltd.-   2) Acrylic resin: SK Dyne 1310 (32 to 34% insolubles, with the    balance being solvent), produced by Soken Chemical & Engineering    Co., Ltd.-   3) Surface treatment agent for powder: carbon functional silane    (KBM-3101, produced by Shin-Etsu Chemical Co., Ltd.)-   4) Heat conductive filler: alumina powder (AS30, produced by Showa    Denko K.K.)-   5) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15 (a soft magnetic metal powder composed of round    particles) by Daito Steel Co., Ltd.-   6) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15F (a soft magnetic metal powder composed of flaky    particles) by Daito Steel Co., Ltd.    Fabrication and Performance Evaluation of Thermosoftening    Electromagnetic Wave Absorbing Heat Dissipating Sheet

The starting materials were added to a homogenizer in the proportionsshown in Table 1 and agitated at room temperature for one hour to effectmixture. Using a comma coater, the resulting mixture was applied onto arelease agent-coated PET film, then heated in a 100° C. atmosphere for10 minutes to remove the solvent (volatiles), thereby forming a sheethaving a width of 300 mm and a thickness of 0.5 mm.

Samples were punched in a given shape from the resulting thermosofteningelectromagnetic wave absorbing heat dissipating sheet. The PET film waspeeled off each sample, following which noise attenuation, plasticity,thermal conductivity, thermal resistance, viscosity and the heatsoftening temperature were measured as described below.

1) Noise Attenuation

The measurement method is illustrated in the block diagram shown in FIG.1.

A personal computer (PC) 2 in which a thermosoftening electromagnet waveabsorbing heat dissipating sheet of the invention (30 mm wide, 30 mmlong, 0.5 mm thick) had been inserted between the CPU (operatingfrequency, 533 MHz) and an aluminum heat sink was placed in anelectromagnetic anechoic chamber 1. A receiving antenna 3 was positionedthree meters away from the PC 2 to comply with 3-meter testing accordingto the Federal Communication Commission (FCC). Also shown in FIG. 1 area display 4 and a keyboard 5. The PC 2 was started up, and the noisegenerated by the PC 2 was measured with an EMI receiver (spectrumanalyzer) 7 located within a shielded room 6 and connected to thereceiving antenna 3. During the test, the power on the display 4connected to the PC 2 was turned off to prevent noise reception from thedisplay 4.

2) Plasticity

Plasticity was measured in accordance with the plasticity test describedin JIS K-6249.

3) Thermal Conductivity

Measurement was carried out with a thermal conductivity meter (QTM-500,made by Kyoto Denki).

4) Thermal Resistance

A 0.5 mm thick sample punched from the sheet in the shape of a TO-3transistor was placed between a transistor (2SD923, from Fuji ElectricCo., Ltd.) and a heat sink (FBA-150-PS, OS Co., Ltd.), and a compressiveload of 300 gf/cm² was applied. The heat sink was placed in aconstant-temperature water bath and held at 60° C. Next, 10V, 3 A powerwas fed to the transistor, the temperatures of thermocouples embedded inthe transistor (temperature T₁) and the heat sink (temperature T₂) weremeasured after 5 minutes, and the thermal resistance Rs (in ° C./W) ofthe sample was computed as follows.Rs=(T ₁ −T ₂)/305) Viscosity

The viscosity was measured using an ARES viscoelastic measurement system(Rheometric Scientific).

6) Thermosoftening Point

Measured using the Vicat softening temperature test method described inJIS K 7206.

In addition, the sheet formability, flexibility, tack and handleabilityof the above thermosoftening electromagnetic wave absorbing heatdissipating sheets were each rated as “excellent,” “good,” “fair” or“poor” based on the following criteria. The results are presented inTable 1.

Sheet Formability: The extrudablity was evaluated.

-   Flexibility: Rated according to the degree of crack formation when    the sheet was bent 90°.-   Tack: As shown in FIG. 2, the heat dissipating sheet 12 was attached    to the surface of a heat sink 11. This arrangement was held up in    the air for 5 minutes with the heat dissipating sheet 12 on the    bottom, and the tack was rated based on whether the sheet peeled and    fell off.-   Handleability: The ease of manually installing the sheet on the heat    sink was evaluated.

TABLE 1 Starting materials Example Example Example Example (parts byweight) 1 2 3 4 Paraffin Wax 115 0 50 0 50 Paraffin Wax 130 50 0 50 0 SKDyne 1310 150 150 150 150 KBM-3103 5 5 5 5 AS30 0 600 0 300 PMIC-15 900900 0 0 PMIC-15F 0 0 350 350 Noise attenuation at 1 −11.2 −8.8 −13.5−12.1 GHz (dB) Plasticity at 25° C. 310 500 250 300 Thermal conductivity1.3 3.8 0.9 1.8 (W/mK) Thermal resistance at 0.09 0.03 0.10 0.06 60° C.(° C./W) Viscosity at 80° C. 5 × 10³ 3 × 10⁴ 2 × 10³ 1 × 10⁴ (Pa · s)Thermosoftening point 40-80 40-80 40-80 40-80 (° C.) Sheet formabilitygood good good good Flexibility excellent fair excellent good Tackexcellent good excellent excellent Handleability fair fair fair fair

Examples 5 to 8

Fluorocarbon resin-based electromagnetic wave absorbing heat conductivecompositions composed of a mixture of primarily fluorocarbon resin andelectromagnetic wave absorbing fillers and having a softening point ofat least 40° C. were formed as described below into thermosofteningelectromagnetic wave absorbing heat dissipating sheets.

In each case, a liquid fluorocarbon resin was used as the resinconstituent of the fluorocarbon resin-based electromagnetic waveabsorbing heat conductive composition and a polyvinylidenefluoride/hexafluoropropylene/tetra-fluoroethylene ternary resin was usedas the thermosoftening constituent. Another ingredient included in thecompositions was carbon functional silane, which was used as a surfacetreatment agent for the electromagnetic wave absorbing filler and theheat conductive filler. The starting materials from which thecompositions were formulated are listed below.

Starting Materials

-   1) Kynar 9301 (heat softening temperature, 80° C.) manufactured by    Daikin Industries, Ltd.-   2) Liquid fluorocarbon resin: G101 (Daikin Industries, Ltd.)-   3) Surface treatment agent for powder: carbon functional silane    (KBM-3101, produced by Shin-Etsu Chemical Co., Ltd.)-   4) Heat conductive filler: alumina powder (AS30, produced by Showa    Denko K.K.)-   5) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15 (a soft magnetic metal powder composed of round    particles) by Daito Steel Co., Ltd.-   6) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15F (a soft magnetic metal powder composed of flaky    particles) by Daito Steel Co., Ltd.    Fabrication and Performance Evaluation of Thermosoftening    Electromagnetic Wave Absorbing Heat Dissipating Sheet

The starting materials were stirred and mixed in a kneader in theproportions shown in Table 2. Using an extruder, the resulting compoundwas extruded as a 300 mm wide, 0.5 mm thick sheet onto a PET film.

Samples were punched in a given shape from the resulting thermosofteningelectromagnetic wave absorbing heat dissipating sheet. The PET film waspeeled off each sample, following which noise attenuation, plasticity,thermal conductivity, thermal resistance, viscosity and the heatsoftening temperature were measured in the same way as in Example 1. Inaddition, the sheet formability, flexibility, tack and handleability ofeach sheet were evaluated as in Example 1. The results are presented inTable 2 below.

TABLE 2 Starting materials Example Example Example Example (parts byweight) 5 6 7 8 Kynar 9301 0 100 0 100 G101 200 100 200 100 KBM-3103 5 55 5 AS30 0 600 0 300 PMIC-15 1000 1000 0 0 PMIC-15F 0 0 450 450 Noiseattenuation at 1 −13.2 −9.8 −14.5 −13.1 GHz (dB) Plasticity at 25° C.310 500 250 300 Thermal conductivity 1.7 4.4 1.2 2.1 (W/mK) Thermalresistance at 0.09 0.03 0.10 0.06 60° C. (° C./W) Viscosity at 80° C. 5× 10³ 3 × 10⁴ 2 × 10³ 1 × 10⁴ (Pa · s) Thermosoftening point 40-80 40-8040-80 40-80 (° C.) Sheet formability good good good good Flexibilityexcellent fair excellent good Tack excellent good excellent excellentHandleability fair fair fair fair

Examples 9 to 18, and Comparative Example 1

Silicone-based electromagnetic wave absorbing heat conductivecompositions composed of a mixture of primarily silicone resin andelectromagnetic wave absorbing fillers and having a softening point ofat least 40° C. were formed as described below into thermosofteningelectromagnetic wave absorbing heat dissipating sheets.

In each case, a methyl phenyl silicone resin which is a copolymerobtained by combining the structural units CH₃SiO_(3/2), (CH₃)₂SiO,C₆H₅SiO_(3/2), (C₆H₅) (CH₃)SiO and (C₆H₅)₂SiO was used as thethermosoftening constituent in the silicone-based electromagnetic waveabsorbing heat conductive composition. Two vinyl group-bearingdimethylpolysiloxanes of differing viscosities were used as the matrixconstituents. The composition also included, as a surface treatmentagent for the electromagnetic wave absorbing filler and the heatconductive filler, an organopolysiloxane containing silicon atom-bondedalkoxy groups of general formula (1) below

In the formula, R¹ is CH₃ or OH; R² is Si(OCH₃)₃, Si(OC₂H₅)₃, Si(CH₃)₂OHor Si(CH₃)₂NH₂; and m is any integer from 1 to 100.

To improve peelability from the liner at the time of sheet installation,dimethyldiphenylpolysiloxane was used as an internal release agent. Thestarting materials from which the compositions were formulated arelisted below.

Starting Materials

-   1) Thermosoftening ingredients: methyl phenyl silicone resins    (copolymers obtained by combining the structural units CH₃SiO_(3/2),    (CH₃)₂SiO, C₆H₅SiO_(3/2), (C₆H₅)(CH₃)SiO and (C₆H₅)₂SiO). Resins    having softening temperatures of 40° C. (Resin A) and 60° C.    (Resin B) were prepared and used.-   2) Matrix ingredients: Two types of vinyl group-bearing    dimethylpolysiloxane were used.    -   High-viscosity ingredient: uncured rubber (KE-76 VBS, produced        by Shin-Etsu Chemical Co., Ltd.)    -   Low-viscosity ingredient: 30,000 cSt vinyl group-bearing        dimethylpolysiloxane oil (Shin-Etsu Chemical Co., Ltd.)-   3) Surface treatment agent for powder: The silicon atom-bonded    alkoxy group-bearing organopolysiloxane    (CH₃)₃SiO[Si(CH₃)₂O]₃₀Si(OCH₃)₃ produced by Shin-Etsu Chemical Co.,    Ltd.-   4) Internal release agent: Dimethyldiphenylpolysiloxane (KF-54,    produced by Shin-Etsu Chemical Co., Ltd.)-   5) Heat conductive filler: alumina powder (AS30, produced by Showa    Denko K.K.)-   6) Heat conductive filler: aluminum nitride powder (UM, produced by    Toyo Aluminum K.K.)-   7) Heat conductive filler: silicon carbide powder (GP#1000, produced    by Shinano Electric Refining Co., Ltd.)-   8) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15 (a soft magnetic metal powder composed of round    particles) by Daito Steel Co., Ltd.-   9) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15F (a soft magnetic metal powder composed of flaky    particles) by Daito Steel Co., Ltd.-   10) Electromagnetic wave absorbing filler: Mn—Zn ferrite produced    under the trade name BSF547 (a soft magnetic oxide powder composed    of flaky particles) by Toda Kogyo Corporation-   11) Electromagnetic wave absorbing filler: Fe—Ni produced under the    trade name MHT Permalloy PC (a soft magnetic metal powder composed    of round particles) by Mitsubishi Steel Mfg. Co., Ltd.-   12) Electromagnetic wave absorbing filler: Fe—Cr—Si produced under    the trade name MHT410L-3Si (a soft magnetic metal powder composed of    round particles) by Mitsubishi Steel Mfg. Co., Ltd.

To improve reworkability, in an example where the silicone matrix of thethermosoftening electromagnetic absorbing heat dissipating sheet iscrosslinked by the heat generated from operation of the heat generatingelectronic component (Example 18), an organohydrogenpolysiloxane bearingat least two hydrogen atoms bonded to a silicon atom on the molecule, aplatinum group metal catalyst and an acetylene alcohol-based reactionregulator were also added and admixed.

Fabrication and Performance Evaluation of ThermosofteningElectromagnetic Wave Absorbing Heat Dissipating Sheet

The starting materials were added to a planetary mixer in theproportions shown in Tables 3 and 4, and stirred at 120° C. for twohours to effect mixture. The mixture was then deaerated and mixed atroom temperature in a two-roll mill. Using an extruder, the resultingcompound was extruded as a 100 mm wide, 0.5 mm thick sheet. In Example18 in which the silicone matrix was crosslinked, extrusion of thecompound as a 100 mm wide, 0.5 mm thick sheet was preceded by the roomtemperature addition and admixture in the two-roll mill of theorganohydrogenpolysiloxane bearing at least two hydrogen atoms bonded toa silicon atom on the molecule, the platinum group metal catalyst andthe acetylene alcohol-based reaction regulator.

Samples were punched in a given shape from the resulting thermosofteningelectromagnetic wave absorbing heat dissipating sheets, following whichnoise attenuation, plasticity, thermal conductivity, thermal resistance,viscosity and the heat softening temperature were measured in the sameway as in Example 1. In addition, the sheet formability, flexibility,tack and handleability of each sheet were evaluated as in Example 1. Theresults are presented below in Tables 3 and 4.

TABLE 3 Starting materials Example Example Example Example ExampleExample (parts by weight) 9 10 11 12 13 14 Resin A 25 25 25 25 25 25Resin B 0 0 0 0 0 0 KE-76VBS 10 10 10 10 10 10 30,000 cSt vinyl-bearing40 40 40 40 40 40 dimethylpolysiloxane oil Silicon atom-bonded alkoxy-20 20 20 20 20 20 bearing organopolysiloxane KF-54 5 5 5 5 5 5 AS30 0400 0 0 0 0 UM 0 0 400 0 0 0 GP#1000 0 0 0 400 0 0 PMIC-15 900 900 900900 0 0 PMIC-15F 0 0 0 0 350 0 BSF547 0 0 0 0 0 900 Noise attenuation at1 GHz (dB) −11.2 −10.1 −10.5 −9.8 −13.5 −8.8 Plasticity at 25° C. 310410 450 460 260 300 Thermal conductivity (W/mK) 1.1 2.5 3.6 2.7 0.8 2.4Thermal resistance at 60° C. (° C./W) 0.09 0.06 0.04 0.09 0.10 0.06Viscosity at 80° C. (Pa · s) 2 × 10³ 1 × 10⁴ 2 × 10⁴ 3 × 10⁴ 1 × 10³ 3 ×10³ Thermosoftening point (° C.) 40-60 40-60 40-60 40-60 40-60 40-60Sheet formability good good good good good good Flexibility excellentfair fair fair excellent good Tack good fair good good good fairHandleability good fair fair fair fair good

TABLE 4 Starting materials Example Example Example Example Comparative(parts by weight) 15 16 17 18 Example 1 Resin A 25 0 25 25 25 Resin B 025 0 0 0 KE-76VBS 25 10 10 10 10 30,000 cSt vinyl-bearing 40 40 40 40 40dimethylpolysiloxane oil Silicon atom-bonded alkoxy- 10 20 20 20 20bearing organopolysiloxane KF-54 5 5 5 5 5 Organohydrogenpolysiloxane 00 0 2.0 0 Platinum group metal catalyst 0 0 0 0.2 0 Acetylenealcohol-based 0 0 0 0.4 0 reaction regulator AS30 0 0 0 400 1200 UM 0 00 0 0 GP#1000 0 0 0 0 0 PMIC-15 900 900 400 600 0 PMIC-15F 0 0 150 0 0BSF547 0 0 0 300 0 Noise attenuation at 1 GHz (dB) −10.7 −11.1 −11.2−10.7 0 Plasticity at 25° C. 410 330 290 410 350 Thermal conductivity(W/mK) 1.2 1.3 1.0 2.5 4.1 Thermal resistance at 60° C. (° C./W) 0.040.10 0.08 0.06 0.03 Viscosity at 80° C. (Pa · s) 8 × 10³ 5 × 10³ 5 × 10³1 × 10⁴ 5 × 10⁴ Thermosoftening point (° C.) 40-60 60-80 40-60 40-6040-60 Sheet formability good good fair good good Flexibility goodexcellent excellent good good Tack good good good fair goodHandleability good fair good good good

To evaluate the reworkability, the thermosoftening electromagnetic waveabsorbing heat dissipating sheet was set between a heat sink and a CPU,and the CPU was operated for 3 hours, following which the ease ofremoving the sheet from the heat sink and the CPU was determined. InExample 18, the fact that the sheet was crosslinked improved thereworkability. Remnants of the sheet that remained on the CPU and heatsink were easily and completely removed by wiping with a dry cloth.

Examples 19 to 31, and Comparative Examples 2 to 4

Polyolefin-based electromagnetic wave absorbing heat conductivecompositions composed of a mixture of primarily polyolefin andelectromagnetic wave absorbing fillers and having a softening point ofat least 40° C. were formed as described below into thermosofteningelectromagnetic wave absorbing heat dissipating sheets.

In each of these examples, an α-olefin of general formula (2)CH₂═CH(CH₂)_(n)CH₃  (2),wherein n is 16 to 50, was used as the thermosoftening ingredient in thepolyolefin-based electromagnetic wave absorbing heat conductivecomposition. The ethylene/α-olefin/non-conjugated polyene randomcopolymer rubbers of general formulas (3) and (4) below were used as thematrix constituents.

In formula (3), the letter x represents an integer from 0 to 10, R³ is ahydrogen or an alkyl having 1 to 10 carbons, and R⁴ is a hydrogen or analkyl having 1 to 5 carbons. In formula (4), R⁵ is a hydrogen or analkyl having 1 to 10 carbons.

To impart the sheet with flexibility and tack, the compositions alsoincluded polymers of general formula (5)[(CH₂—CH₂)_(X)—(CH₂—CRH)_(Y)]_(P)  (5)and of differing viscosities. In formula (5), R is an alkyl representedby the formula C_(W)H_(2W+1); and X, Y, P and W are integers, such thatgenerally X is 1 to 100, Y is 5 to 100, P is 5 to 500 and W is 1 to 10.The starting materials from which the compositions were formulated arelisted below.Starting Materials

-   1) Matrix ingredients: Ethylene/α-olefin/non-conjugated polyene    random copolymers    -   EPT-PX055 (Mooney viscosity at 100° C., 8; ethylene content, 58        wt %) produced by Mitsui Chemicals, Inc.    -   EPT-4010 (Mooney viscosity at 100° C., 8; ethylene content, 65        wt %) produced by Mitsui Chemicals, Inc.    -   EPT-4021 (Mooney viscosity at 100° C., 24; ethylene content, 67        wt %) produced by Mitsui Chemicals, Inc.    -   EPT-X3012P (Mooney viscosity at 100° C., 15; ethylene content,        70 wt %) produced by Mitsui Chemicals, Inc.-   2) Matrix ingredients: ethylene/α-olefin copolymers    -   Lucant HC40 (viscosity at 25° C., 350 cSt) produced by Mitsui        Chemicals, Inc.    -   Lucant HC3000X (viscosity at 25° C., 25,000 cSt) produced by        Mitsui Chemicals, Inc.    -   Lucant HC10 (viscosity at 25° C., 140 cSt) produced by Mitsui        Chemicals, Inc.-   3) Thermosoftening ingredients: α-olefins    -   DIALEN 30 (n=30 to 40) produced by Mitsubishi Chemical Corp.    -   DIALEN 208 (n=17 to 25) produced by Mitsubishi Chemical Corp.-   4) Heat conductive filler: alumina powder (AS30, produced by Showa    Denko K.K.)-   5) Heat conductive filler: aluminum nitride powder (UM, produced by    Toyo Aluminum K.K.)-   6) Heat conductive filler: silicon carbide powder (GP#1000, produced    by Shinano Electric Refining Co., Ltd.)-   7) Electromagnetic wave absorbing filler: Fe—produced under the    trade name PMIC-15 (a soft magnetic metal powder composed of round    particles) by Daito Steel Co., Ltd.-   8) Electromagnetic wave absorbing filler: Fe—Cr produced under the    trade name PMIC-15F (a soft magnetic metal powder composed of flaky    particles) by Daito Steel Co., Ltd.-   9) Electromagnetic wave absorbing filler: Mn—Zn ferrite produced    under the trade name BSF547 (a soft magnetic oxide powder composed    of flaky particles) by Toda Kogyo Corporation-   10) Electromagnetic wave absorbing filler: Fe—Ni produced under the    trade name MHT Permalloy PC (a soft magnetic metal powder composed    of round particles) by Mitsubishi Steel Mfg. Co., Ltd.-   12) Electromagnetic wave absorbing filler: Fe—Cr—Si produced under    the trade name MHT410L-3Si (a soft magnetic metal powder composed of    round particles) by Mitsubishi Steel Mfg. Co., Ltd.-   12) Surface treatment agent for powder: carbon functional silane    (KBM-3101, produced by Shin-Etsu Chemical Co., Ltd.)    Fabrication and Performance Evaluation of Thermosoftening    Electromagnetic Wave Absorbing Heat Dissipating Sheet

The starting materials were added to a planetary mixer in theproportions shown in Tables 5 and 6, and stirred at 100° C. for twohours to effect mixture. The mixture was then deaerated and mixed atroom temperature in a two-roll mill. Using an extruder, the resultingcompound was extruded as a 100 mm wide, 0.5 mm thick sheet.

Samples were punched in a given shape from the resulting thermosofteningelectromagnetic wave absorbing heat dissipating sheets, following whichnoise attenuation, plasticity, thermal conductivity, thermal resistance,viscosity and the heat softening temperature were measured in the sameway as in Example 1. In addition, the sheet formability, flexibility,tack and handleability of each sheet were evaluated as in Example 1. Theresults are presented below in Tables 5 and 6.

TABLE 5 Starting materials Example Example Example Example ExampleExample Example Example (parts by weight) 19 20 21 22 23 24 25 26EPT-PX055 20 0 0 0 20 10 20 20 EPT-4010 0 20 0 0 0 10 0 0 EPT-4021 0 020 0 0 0 0 0 EPT-X3012P 0 0 0 20 0 0 0 0 Lucant HC10 0 0 0 0 0 0 5 10Lucant HC3000X 30 30 30 30 30 30 25 30 DIALEN 30 20 20 20 20 20 20 20 10DIALEN 208 30 30 30 30 30 30 30 30 KBM-3103 6 6 6 6 6 6 6 6 AS30 0 0 0 00 400 400 400 UM 0 400 0 0 0 0 0 0 GP#1000 0 0 400 0 0 0 0 0 PMIC-15 900900 900 0 0 900 900 900 PMIC-15F 0 0 0 200 0 0 0 0 BSF547 0 0 0 0 800 00 0 Noise attenuation −11.0 −11.5 −11.4 −12.2 −5.5 −11.0 −11.2 −11.1 at1 GHz (dB) Plasticity at 25° C. 340 360 450 500 390 420 290 310 Thermalconductivity 0.9 2.3 2.5 1.0 1.3 2.4 2.3 2.2 (W/mK) Thermal resistance0.09 0.06 0.06 0.09 0.10 0.06 0.05 0.04 at 60° C. (° C./W) Viscosity at80° C. 1 × 10⁴ 5 × 10³ 6 × 10⁴ 8 × 10⁴ 3 × 10⁴ 3 × 10⁴ 2 × 10⁴ 8 × 10³(Pa · s) Thermosoftening point 40-80 40-80 40-80 40-80 40-80 40-80 40-8040-80 (° C.) Sheet formability good good good good good good good goodFlexibility fair fair fair fair fair fair good good Tack fair fair fairfair fair fair fair good Handleability fair fair fair fair good goodgood fair

TABLE 6 Comparative Comparative Comparative Starting materials ExampleExample Example Example Example Example Example Example (parts byweight) 27 28 29 30 31 2 3 4 EPT-4010 10 10 10 10 10 0 0 10 EPT-PX055 1010 10 10 10 20 20 10 Lucant HC40 10 10 10 10 10 0 0 0 Lucant HC3000X 2020 20 20 30 30 30 30 DIALEN 30 20 20 20 20 20 20 20 20 DIALEN 208 30 3030 30 30 30 30 30 KBM-3103 6 6 6 6 6 6 6 6 AS30 400 400 300 300 500 01200 0 PMIC-15 600 0 0 300 100 1000 0 0 PMIC-15F 150 0 0 50 200 0 0 0MHT Permalloy PC 0 900 0 450 100 0 0 0 MHT410L-3Si 0 0 1000 200 100 0 00 Noise attenuation −14.0 −12.0 −11.5 −15.0 −15.6 −13.0 0 0 at 1 GHz(dB) Plasticity at 25° C. 400 430 440 440 440 400 310 not measurableThermal conductivity 2.4 2.3 2.2 2.4 2.3 1.8 3.0 0.3 (W/mK) Thermalresistance 0.03 0.04 0.05 0.03 0.05 0.09 0.03 1.2 at 60° C. (° C./W)Viscosity at 80° C. 1.5 × 10³ 1 × 10⁴ 7 × 10² 2 × 10⁴ 8 × 10³ 5 × 10⁴ 8× 10³ 40 (Pa · s) Thermosoftening point 40-80 40-80 40-80 40-80 40-8040-80 40-80 not (° C.) measurable Sheet formability excellent good goodgood good good good poor Flexibility excellent excellent good excellentgood good good poor Tack excellent excellent excellent excellent goodgood good poor Handleability excellent excellent excellent excellentgood good good poor

Comparative Examples 5 to 8

For the sake of comparison, the results of physical propertymeasurements and handleability tests carried out on commerciallyavailable silicone rubber heat dissipating sheets (0.5 mm thick,Comparative Examples 5 to 7) and a commercially available heatdissipating grease (Comparative Example 8) are shown below in Table 7.

TABLE 7 Com- Com- Com- Com- parative parative parative parative Example5 Example 6 Example 7 Example 8 Noise attenuation 0 0 0 0 at 1 GHz (dB)Thermal conductivity 2.0 3.0 4.0 4.7 (W/mK) Thermal resistance 0.58 0.470.27 0.03 at 60° C. (° C./W) Handleability fair fair good poor

As is apparent from the above results, the thermosofteningelectromagnetic wave absorbing heat dissipating sheets obtained in theexamples of the invention have contact thermal resistances so much lowerthan those of silicone rubber heat dissipating sheets of comparablethermal conductivity as to be essentially negligible. Because of theirlow thermal resistances, the sheets of the invention had excellent heatdissipating performances and effectively dissipated the heat generatedby electronic components. The sheets obtained in the examples accordingto the invention also exhibited a high noise attenuation, and thus hadan excellent ability to absorb electromagnetic waves.

The invention thus provides both electromagnetic wave absorbing heatconductive compositions having an excellent ability to dissipate heatand excellent electromagnetic wave absorbing properties, and alsothermosoftening electromagnetic wave absorbing heat dissipating sheetsproduced from such compositions.

1. An electromagnetic wave absorbing heat dissipating device,comprising: a heat generating electronic component which, when operated,generates heat and reaches a temperature higher than room temperatureand which generates electromagnetic waves; a heat dissipating componentwhich faces the heat generating electronic component; and anelectromagnetic wave absorbing, heat conductive composition positionedtherebetween, said composition comprising an organic binder componentwhich is a mixture of copolymers of polymers containing RSiO_(3/2) unitswith R₂SiO units, wherein R is methyl, phenyl or combinations thereof,with linear polysiloxanes, and an electromagnetic wave absorbing filler,wherein, when the device is in operation whereby the heat generatingelectronic component generates heat, and reaches a temperature higherthan room temperature, and generates electromagnetic waves, the heatcausing the organic binder component to fluidize as a low viscositymaterial, or to soften or to melt to the extent that at least a surfaceof the composition is fluidized with the result that the compositionflows and substantially fills any gaps between the heat generatingelectronic component and the heat-dissipating component, saidcomposition having a thermal conductivity of at least 0.5 W/mK and aviscosity at 80° C. of 1×10² to 1×10⁵ Pa·s.
 2. The device of claim 1,wherein the composition is curable by a crosslinking reaction.
 3. Thedevice of claim 1, wherein the composition further comprises a lowmelting substance having a melting point of 40 to 100° C., a heatflowable substance which softens or acquires a low viscosity and becomesfluid at a temperature of 40 to 100° C., or a syrupy substance at atemperature of 40 to 100° C.
 4. The device of claim 1, wherein thecomposition further comprises at least one component having a meltingpoint of 40 to 100° C. selected from the group consisting of α-olefins,waxes and silicone resins.
 5. The device of claim 1, wherein thecomposition further comprises a heat conductive filler, and wherein thetemperature that effectively has a viscosity lowering, softening ormelting effect on the organic binder is at least 40° C.
 6. The device ofclaim 1, wherein the electromagnetic wave absorbing filler is at leastone substance selected from the group consisting of ferromagnetic metalpowders and ferromagnetic oxide powders.
 7. The device of claim 5,wherein the heat conductive filler is at least one material selectedfrom the group consisting of nonmagnetic metals, metal oxides, metalnitrides and silicon carbide.
 8. The device of claim 5, wherein theelectromagnetic wave absorbing filler and the heat conductive fillereach range in particles size from 0.1 to 100 μm.